198 POPULAR LECTURES AND ADDRESSES. 



to the exciting beam of light to be very completely 

 polarised in the plane through my eye and the 

 exciting beam. It consists of light-vibrations in 

 one definite direction, and that, as finally demon- 

 strated by Professor Stokes, it seems to me beyond 

 all doubt, through reasoning on this phenomenon 

 of polarisation, 1 which he had observed in various 



1 Extract from Professor Stokes's paper " On the Change of 

 Refrangibility of Light," read before the Royal Society, May 27th, 

 1852, and published in the Transactions for that date : 



" 183. Now this result appears to me to have no remote 

 bearing on the question of the directions of the vibration in polarised 

 light. So long as the suspended particles are large compared with 

 the waves of light, reflection takes place as it would from a portion 

 of the surface of a large solid immersed in the fluid, and no con- 

 clusion can be drawn either way. But if the diameters of the 

 particles be small compared with the length of a wave of light, it 

 seems plain that the vibrations in a reflected ray cannot be perpen- 

 dicular to the vibrations in the incident ray. Let us suppose for the 

 present, that in the case of the beams actually observed, the sus- 

 pended particles were small compared with the length of a wave 

 of light. Observation showed that the reflected ray was polarised. 

 Now all the appearances presented by a plane polarised ray are 

 symmetrical with respect to the plane of polarisation. Hence we 

 have two directions to choose between for the direction of the 

 vibrations in the reflected ray, namely, that of the incident ray, and 

 a direction perpendicular to both the incident and the reflected rays. 

 The former would be necessarily perpendicular to the directions of 

 vibration in the incident ray, and therefore we are obliged to choose 

 the latter, and consequently to suppose that the vibrations of 



